There are many references to java fern turning brown on the web, including this forum... maybe too many. I can't seem to find definitive advice.

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Last October I bought a huge bundle of mature java fern (I don't know the exact variety). I read enough to learn not to plant it in substrate. I attached it to driftwood pieces. After a few weeks of acclimating, it took off. It was vibrant green, shot out fernlets all over the place, and was doing well. (My tank then was newly- but well-cycled using fishless method).

For a couple months it has been progressively turning brown and dying back. This isn't just browning of a few old leaves. Serious plant thinning and dying. The only thing I can think of I've changed is going quite a while between water changes. Before I get hammered, let me explain why.

First I have some health issues that make frequent water changes challenging sometimes, but a big reason I let it go much longer between water changes is because the nitrates never increased over 5 ppm (ammonia 0, nitrites 0, hardness about 7.8 ). I figured my plants were to thank and must actually benefit from some nitrates. This, along with the fact that my fish seem very happy and healthy (the Acei even bred for the first time) made me decide to wait to see if nitrates go up above 5. They didn't.

By the way my anubias is doing well still. My lighting is T-8, one bulb is daylight from Lowe's and the other is 10,000K Zoo Med bulb. I have them on a timer for about 11-12 hours each day, little-to-no natural sunlight gets in. My water test kit can't test for phosphorous, potassium, etc. I thought java fern would not require fertilizing, so I haven't done that.

Could infrequent water changes adversely effect the plants even though nitrates are so low already? Anybody have advice on how to save the java fern? I really liked it when it was doing well and I hate to lose what's left of the mature plants.

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Fernlets, as you call them, is not a sign that java fern is doing well. It's a sign of leaf damage and the plant trying to save it's genes by creating better adapted versions of itself that can float away and attach somewhere.
Did the rhizome grow at all? Or new leaves grow from the primary rhizome? Or always from the existing leaves?

I didn't know that "fernlets" did not necessarily indicate a healthy plant. (BTW, I'm happy to learn the correct term if you want to enlighten me.)

To be honest, the plant was so thick with such large leaves, I never really paid attention to what it was doing at the rhizome. Now that so much of the leaves have died off, it's easier to notice that there are some new "shoots" of growth directly from the rhizome. I wouldn't say it's bursting with life, but there are some. I wouldn't say the rhizome itself has grown. Essentially, where I had thick bundles of java fern before, now I have thin bundles with about half the overall mass.

I must be doing something wrong, I just don't know what. After reading what you've said, it sounds like maybe they were always on the decline, but just took a long while to show the drastic results. Either way, I'd really like to save what's left if possible, but I don't know where to start.

(This is my first African Cichlid tank and my first attempt at live aquatic plants of any kind. That's one reason I started with these plants that are supposed to be tough. It's a bit discouraging that I can't even get java fern to thrive.)

I don't know the real term for the daughter plants... only colloquial names. Suckers, daughter plants, plantlets and now, "fernlets". A good name for them really...

I've had a whole tank of java fern (the windelov variety) suddenly melt, so yes... trouble can be brewing for quite some time before you even notice. Nothing more maddening than watching something that took a year to grow - melt within a week!

My guess is that the plant didn't like the change from tank to tank. If you've had it a while and it was doing fine, then something has suddenly changed (drastically) in the tank and you are simply unaware of it (or already solved for it). One aquarist with a green thumb once told me to keep a log book if I ever really wanted to be a "pro" as plants have a maddening habit of reacting to owner's errors about 1 week after you've completely forgotten about whatever it was that you "fixed".

If it started to look suspicious back in November and it's just taken until now for the plant to croak, then my advice is simply pull out all dead leaves and watch the rhizome for root vs new growth. If it starts to grow back, then do nothing... patience grows a planted tank... impatience grows algae

A member of our local club just wrote an article for our newsletter on java fern melting. He had ferns in two different tanks that were almost identical in their set up and maintenance. In one tank the ferns were fine, in the other they occasionally turned brown. The only difference he found was that the healthy ferns were growing on limestone, while the ones that periodically melted were growing on wood. It seems the ferns liked the limestone, or calcium carbonate. If you can increase your KH and GH, the fern would probably benefit, but you'd need to take your fish into account (would they appreciate it? If so, go very slowly and gradually).